1. Field of the Invention
The invention relates to a magnetic-inductive flow meter, with at least one measuring tube for through flow of an electrically conductive medium, with a magnetic field generating apparatus for generating an alternating magnetic field which runs at least also perpendicular to the longitudinal axis of the measuring tube, with at least two measuring electrodes which especially contact the medium, and with an evaluation circuit, the magnetic field generating apparatus having at least one field coil and one coil power supply and the coil power supply preferably having a current controller and preferably one changeover bridge. The invention also relates to a method for operating a magnetic-inductive flow meter of the type as has been described above in particular.
2. Description of Related Art
German Patent Application DE 199 07 864 A1 and corresponding U.S. Pat. No. 6,453,754 B1 disclose a magnetic-inductive flow meter of the above described type. In this known magnetic-inductive flow meter the magnetic field generating apparatus can have one field coil or two field coils. This is why it was stated above that the magnetic field generating apparatus has at least one field coil. In the known magnetic-inductive flow meter, the magnetic field generating apparatus also has a current controller and a changeover bridge. But, because neither a current controller nor a changeover bridge is critical to operation, it was stated above that the magnetic field generating apparatus has preferably one current controller and preferably one changeover bridge.
Magnetic-inductive flow meters have been widely known in the prior art for decades. Reference is made by way of example to the literature citation Technical Flow Rate Measurement by Dr. Eng. K. W. Bonfig, 3rd edition, Vulcan-Verlag Essen, 2002, pp. 123 to 167, and moreover, to the literature citation Principles of Magnetic-Inductive Flow Rate Measurement by Cert. Eng. Friedrich Hoffmann, 3rd ed., 2003, a publication of the company KROHNE Messtechnik GmbH & Co. KG.
The basic principle of a magnetic-inductive flow meter for measuring the flow rate of a flowing medium goes back to Michael Faraday who suggested the use of the principle of electromagnetic induction for measuring the flow velocity of an electrically conductive medium as early as 1832.
According to the Faraday Induction Law, in a flowing, electrically conductive medium which is permeated by a magnetic field, an electrical field intensity arises perpendicular to the flow direction of the medium and perpendicular to the magnetic field. The Faraday Induction Law is used in magnetic-inductive flow meters in that, by means of a magnetic field generating apparatus which has at least one field coil, conventionally two field coils, a magnetic field which changes over time during a measurement process is generated and the magnetic field at least partially permeates the electrically conductive medium which is flowing through a measuring tube. The generated magnetic field has at least one component perpendicular to the longitudinal axis of the measuring tube and perpendicular to the flow direction of the medium.
It was stated at the beginning that the magnetic-inductive flow meter under discussion includes at least one magnetic field generating apparatus “for producing a magnetic field which runs at least also perpendicular to the longitudinal axis of the measuring tube”, while it is pointed out again here that the magnetic field does run preferably perpendicular to the longitudinal axis of the measuring tube and perpendicular to the flow direction of the medium, it is sufficient that one component of the magnetic field runs perpendicular to the longitudinal axis of the measuring tube and perpendicular to the flow direction of the medium.
It was also stated at the beginning that the magnetic-inductive flow meter under discussion includes at least two measuring electrodes which contact especially the medium. These measuring electrodes are used to tap a measurement voltage which has been induced in the flowing medium. Preferably, the virtual connecting line of the two measuring electrodes runs at least essentially perpendicular to the direction of the magnetic field which is permeating the measuring tube perpendicular to the longitudinal axis of the measuring tube, in particular the measuring electrodes can be provided such that their virtual connecting line in fact runs more or less perpendicular to the direction of the magnetic field which permeates the measuring tube.
Finally, it was stated at the beginning that the measuring electrodes can be especially those which contact the medium. In fact, of course, the electrical field intensity generated by induction in the flowing electrically conductive medium can be tapped as measurement voltage by measuring electrodes which are directly, therefore conductively in contact with the medium. But, there are also magnetic-inductive flow meters in which the measurement voltage is not tapped by measuring electrodes which are directly in contact with the medium, therefore not by measuring electrodes which are conductively in contact with the medium, rather the measurement voltage is capacitively detected.
Initially, magnetic-inductive flow meters were operated in the industrial domain with an alternating magnetic field. For reasons of costs, the field coil or the field coils were connected to the existing AC voltage grid so that the magnetic field changes its field intensity essentially sinusoidal. But, the operation of magnetic-inductive flow meters with a magnetic field which essentially sinusoidally changes its field intensity has disadvantages (see, German Patent Application DE 199 07 864 A1, column 1, line 53 to column 2, line 13).
Since the mid-1970s magnetic-inductive flow meters which operate with a switched constant magnetic field in which therefore a switched direct current is used as the coil current have become increasingly popular. If a switched constant magnetic field is used, disadvantages are avoided which occur when a magnetic field is used whose field intensity changes essentially sinusoidally. But, there are also problems when a switched constant magnetic field is used (see in this respect German Patent Application DE 199 07 864 A1, column 2, lines 18 to 41).
The object of the invention which is described in German Patent Application DE 199 07 864 A1 was to configure and develop the known magnetic-inductive flow meter which operates with a switched constant magnetic field such that the explained, system-induced changeover phases are shorter than in the magnetic-inductive flow meters which were known previously in the prior art so that the field frequency, therefore the frequency with which the constant magnetic field is changed over, can be increased (see, German Patent Application DE 199 07 864 A1, column 2, lines 42 to 49).
In particular, in the known magnetic-inductive flow meter there is a boosting current source and a boosting current can be fed into the field coil or into the field coils by means of the boosting current source immediately at the start of each half wave of the coil current which is present as a switched direct current (see, German Patent Application DE 199 07 864 A1, column 2, lines 50 to 57, claim 1, and also the further explanation in column 2, line 58, to column 4, line 7).
Magnetic-inductive flow meters of the type under consideration virtually consist of two functional units. The first functional unit, also called a sensor, includes the measuring tube, the field coil or field coils of the magnetic field generating apparatus and the measuring electrodes. The second functional unit, also called the electronics, includes the coil current supply and the evaluation circuit. Conventionally, the coil current supply and the evaluation circuit are implemented on a printed circuit board or on several printed circuit boards. The first functional unit, therefore the sensor, is generally connected via a cable to the second functional unit, therefore the electronics.
In magnetic-inductive flow meters in which the electronics are located more or less directly on the sensor, compact versions, the cables are rather short. But, there are also embodiments in which the sensor on the one hand and the electronics on the other are separate, remote versions. Here under certain circumstances a rather long cable, with a length of up to 100 m, between the sensor and the electronics, can be necessary.
In safety-relevant applications of magnetic-inductive flow meters and also in safety-relevant applications of other flow meters, for a long time there has been the desire, often also the necessity, of completely monitoring the serviceability of the flow meter. To date, this has only been conditionally possible in magnetic-inductive flow meters.
The field coil circuit, electrical circuit to which the field coil or field coils and the coil current supply belong can be monitored by the electronics both for interruption and also short circuit. The measuring electrode circuit, the electrical circuit to which the measuring electrodes belong, can be monitored for atypical impedance in certain ranges. If an electrically conductive medium is flowing through a magnetic-inductive flow meter of the type under consideration, by modulation of the coil current both the field coil circuit and also the measuring electrode circuit can also be monitored, specifically in that the signals resulting from a modulation of the coil current are evaluated. But, this does not work when an electrically conductive medium is not flowing through the magnetic-inductive flow meter, if therefore the flow rate is zero.